Contamination of particulate materials (e.g., soils, sediments, sludges, or muds) can arise from hazardous materials being spilled, leaked, discharged, co-processed, or buried at a site, or from intrusion of contaminants from offsite sources. As an example, leaking underground storage tanks have contaminated the soil at sites with petroleum hydrocarbons and lead, and caused similar contamination of adjacent sites through migration in the subsurface. In general, contaminants which may be found in particulate materials include liquids, which may have associated vapors, and solids. Contaminants may be physically or chemically attached to particles, or may be present as a separate phase between particles, such as non-aqueous phase liquids (NAPLs). Liquid contaminants include petroleum hydrocarbons, coal tars, and industrial solvents, while solid contaminants include salts, metals, and organic materials (e.g., explosives, pesticides).
Remediation of contaminated particulate materials implies the removal of contaminants and their impacts for the general protection of health and for the benefit of the environment. For example, remediation at former industrial (“brownfield”) sites may be a prerequisite for redevelopment for residential, commercial, or new industrial use. Remediation is generally subject to an array of regulatory requirements, and also can be based on assessments of human health and ecological risks in light of planned future use.
Site remediation methods addressing contamination can be classified in general terms as: (1) ex situ methods in which the particulate material is displaced and treated or disposed of in a waste facility; and (2) in situ methods in which the particulate material remains in place for treatment. Stabilization, solidification, or containment, while not remediation methods in themselves, may also be used to prevent the contamination from becoming more widespread. Treatment methods for organic contaminants are numerous and varied, with examples including: thermal approaches like incineration, smoldering and thermal desorption; washing/flushing with aqueous solutions or organic solvents; bioremediation, in which microbial activity is stimulated to achieve enhanced biodegradation; and chemical oxidation, typically with an aqueous solution or gas.
Of the thermal remediation methods for particulate materials, incineration is generally the most effective for destroying organic contaminants due to the high temperatures achieved. However, high temperatures also have greater associated fuel costs and tend to degrade native properties of soils and sediments. Due to the range of wastes fed into incinerators, their emissions must also be carefully monitored and controlled (e.g., through temperature and filtration). Thermal desorption, in which organic contaminants are desorbed from particulate materials through the application of heat, is a leading alternative to incineration. Material that is evaporated off of the contaminated soil is collected outside the thermal desorption unit (also referred to as the “thermal desorber” or just “desorber” herein) and either condensed and disposed of or oxidized to its mineral constituents. After treatment, the soil can be returned to its native location.
The thermal desorption process is carried out in a non-combustive environment, although some localized combustion can occur in conventional processes. Combustion during thermal desorption is generally undesired in current practice because it poses an explosion hazard and could lead to regulatory classification as a form of incineration rather than separation. Some units will operate with an oxygen-deficient carrier gas (e.g., a flue gas or an inert gas) or at a vacuum to lower the risk of combustion. Limiting the combustibles in the particulate feed to the unit is also a common practice.
Thermal desorption is a cooler (usually 150-500° C.), less harsh treatment than incineration, but may be ineffective for particulate materials with certain textures (e.g., clays), high moisture contents (e.g., wet soils, sediments) or elevated levels of certain contaminants. The cooler operating temperatures can result in unsatisfactory performance (e.g., failure to remediate or excessive cost) for soils with low-volatility or tightly-bound organic contaminants. Such contaminants can include hydrocarbons heavier than C16 (depending on desorption temperature, treatment time and concentration) and polycyclic aromatic hydrocarbons (PAHs). These limitations pose performance challenges for soils impacted by heavier sources (e.g., diesel fuels, lubricating oils, bunker fuels, crude oils, coal tars).
The output from a thermal desorption process is a dry, hot particulate material with relatively small aggregates. Larger clumps of particulate entering the desorber tend to break down through drying and agitating during treatment, resulting in a product that is closer to a sand or powder. This product is quenched with water to provide cooling, reduce dust and restore more natural properties (e.g., aggregate structure, moisture).
Smoldering, which is a flameless combustion process, is a more recent approach for remediating particulate materials containing organic contamination. In a smoldering remediation process a self-sustaining, flameless combustion front is propagated in a contaminated soil (either in situ or ex situ) resulting in the mineralization of hydrocarbon contaminants. Smoldering differs from flaming combustion as oxidation reactions occur on the surface of a solid or liquid rather than in the gas phase. As the soil can act as an insulator, smoldering can be self-sustaining after ignition (i.e. no external heating is required) provided the soil has sufficient oxygen to maintain combustion and sufficient fuel content. Achieving self-sustaining combustion front propagation is a key feature of existing smoldering technologies and requires a self-sustaining threshold (“SST”) of soil fuel content. The SST substantially limits the applicability of smoldering technologies for treating soils impacted by petroleum hydrocarbons, which often have insufficient fuel content in regions within the soil volume. In addition, the process of propagating a combustion front through soil by forced aeration inherently restricts existing smoldering technologies to soils with preferred properties (e.g., dry, sandy texture). Soils with higher levels of moisture or silt/clay are often untreatable or treatable only with low air supply rates that make the throughput inadequate for commercial use.
Smoldering may be sustained in a particulate material provided sufficient fuel is present. This process occurs naturally, for example, in underground peat fires. However, organic contaminants can also provide sufficient energy for self-sustaining smoldering combustion under the right conditions. Generally, these conditions include high enough contaminant concentrations, a supply of air, adequate retention of heat, and an initial source of heat to ignite the smoldering front. If these conditions are met, smoldering can be used as a process to remediate particulate materials, virtually eliminating all organic contaminants.
Smoldering combustion can be initiated by actively heating a small region of contaminated particulate material below the surface and introducing air once that region has reached ignition temperature (typically 200-500° C.). The heater may then be deactivated, while the air supply is maintained to sustain a smoldering front, which propagates through the bed of particulate material destroying contaminants. Provided there is sufficient fuel for the process in the particulate material, smoldering can be self-sustaining in the sense that no further active heating is required after ignition, as the contaminants themselves supply the heat required for their ongoing destruction.
As stated above, however, the fuel content of the contaminated soil is often inadequate to support propagation of a combustion front through the soil. It has been suggested to additize the soil with an organic fuel source, see U.S. Publication No. 2014/0241806 to Rockwell (incorporated herein in its entirety by reference), but this adds another cost to the remediation process, which is undesirable.
Additionally, the propagation of a combustion front in smoldering processes is slow as heat is transferred through the insulating particulate medium. This mechanism limits throughput to a fraction of that which may be achieved through thermal desorption, e.g., 5-10 tonnes/hour with smoldering versus 20-40 tonnes/hour with thermal desorption. Finally, current smoldering processes are carried out in non-homogeneous contaminated soils and require forced aeration to maintain oxygen levels sufficient for combustion. The heterogeneity of the soil bed (e.g., texture, moisture, compaction) and the heterogeneity of the fuel content in the soil bed tend to lead to uneven propagation of the combustion front in conventional smoldering processes. Regions of contaminated soil can be bypassed or inadequately treated as air flow and combustion occur along preferential pathways in the soil bed.
Remediation technologies are frequently benchmarked by cost to offsite disposal at a waste facility, which relocates contaminated particulate materials to an engineered site for long-term storage. Offsite disposal is often the most economical option for these materials and has a relatively low risk of failure in reaching regulatory criteria at a site. Except when transportation is impractical due to distance or quantity of material, few technologies can routinely compete with its combination of reliability and cost effectiveness. One of the most commonly employed alternatives to offsite disposal for organic contamination is bioremediation, which can cost roughly half the price. While bioremediation provides significant savings over offsite disposal, its application is generally restricted to particulate materials with relatively low levels of contamination, especially in the more refractory hydrocarbons (e.g., hydrocarbons heavier than C16 or PAHs). There is therefore a need for a cost-effective remediation technology that is effective when bioremediation cannot be applied.